With the advent of high speed xerographic reproduction machines, a number of problems have been encountered in attempting to stack the copy sheets produced by the machines. In the presently used devices, sheets entering the stacker are often poorly controlled. The sheet needs to be deformed as it enters the stacker to provide stiffness to prevent it from collapsing as it is pushed into the stack by the trailing edge. Since it is common for sheets discharged from a xerographic reproduction machine to have an electrostatic charge thereon, the trailing edge of the sheet may be electrostatically "floated" and may not clear the path of the succeeding sheet fast enough to prevent jams in the stacker. Further, under severe electrostatic conditions, the lead edge of the sheet being stacked may be attracted to the previous sheet, preventing further movement of the sheet into the stacker without folding the trailing edge of the sheet. To prevent sheets from "floating" in the stacker as they are being delivered, it is common practice to utilize light fingers or strips on top of the stack to hold the sheets down. However, in a xerographic reproduction machine, the copy may be produced by the use of a dry toner material which is thereafter fused to the paper by heat and pressure or the sheet may be developed by a liquid toner. Thus the sheets delivered to the stacker are ordinarily heated or dampened due to the fusing or development process in the machine. In the stacker, the sheet is cooled or dried. In stackers utilizing light finger pressure on the top sheet to prevent floating, the sheets are not controlled relative to flatness with the result that the sheets as they are dried or cooled have a tendency to curl resulting in unacceptable copy.
It is therefore the object of the present invention to provide a stacking mechanism wherein the sheets delivered thereto are acted upon over their entire surface to assure proper positioning thereof in the stack and are held in a planar condition to maintain the entry path clear for subsequent sheets and prevent curling of the sheets as they are cooled or dried.